After more than a decade of rapid growth, interest in wind energy continues to grow. Zhe Chen et al., “A Review of the State of the Art of Power Electronics for Wind Turbines,” IEEE Transactions on Power Electronics, Vol. 24, No. 8, August 2009, provides an overview of the technology used for energy generation from wind, as of 2009. According to the Chen article, early wind generation generally used squirrel-cage induction generators connected directly to the alternating-current (AC) grid, but technical development has shifted towards generation technology that supports variable speeds.
FIG. 1 illustrates the basic elements of a typical variable-speed, wind-based generation system 100. A blade 110 is connected to a doubly-fed induction generator 130 through a gearbox 120. The three-phase stator terminals are coupled to the AC grid through a transformer 150. An AC-AC converter 140 is connected between the rotor terminals and the stator terminals. In more detail, the AC-AC converter includes a rotor-side DC-AC converter 142 and a stator-side AC-DC converter (i.e., a three-phase rectifier) 146 connected together via their respective DC sides. A capacitor 144 is positioned between the two converters, and reduces the variations in the DC-link voltage while providing an energy buffer between the two converters.
The frequency and voltage of the induction generator output vary as the speed of the generator changes. The AC-AC converter allows this variable-voltage, variable-frequency output of the generator to be converted into a fixed-frequency, fixed-voltage output that can be coupled directly to the AC grid. More particularly, the rotor-side converter 142, which is effectively a three-phase inverter circuit, applies a voltage to the rotor windings of the doubly-fed induction generator 130, and controls the rotor currents so that the rotor flux maintains an optimal position relative to the stator flux, for a given torque at the generator shaft. The stator-side converter 146, which is effectively a three-phase rectifier circuit, regulates the voltage of the DC link, at capacitor 144, and generates or absorbs reactive power as needed. An extensive tutorial on the operation of the doubly-fed induction generator in the context of wind power can be found in John Fletcher and Jin Yang, “Introduction to the Doubly-Fed Induction Generator for Wind Power Applications,” Paths to Sustainable Energy, Dr. Artie Ng (Ed.), InTech (2010), available at: http://www.intechopen.com/books/paths-to-sustainable-energy/introduction-to-the-doubly-fed-inductiongenerator-for-wind-power-applications.
The power electronics in a wind generator system, such as the AC-AC converter 140 of FIG. 1, are a costly and critical element. FIG. 2 illustrates a typical bi-directional voltage source inverter circuit 200, which might be used as shown for DC-AC converter 146 in FIG. 1. A similar circuit, but reversed from right to left, can be used as the AC-DC converter 142 in FIG. 1 Each of the three phases from the AC side are connected to the center of a respective bridge leg, at the nodes marked a, b, and c. This six-switch inverter topology typically uses insulated-gate bipolar transistors (IGBTs) for high power-handling capability. The details of its operation and use in AC-AC converters are well known and are described in the Fletcher and Yang reference identified above.
The system illustrated in FIG. 1 comprises a doubly-fed induction generator (DFIG), an AC-AC converter, and a transformer. Other approaches to wind-based power generation systems have been developed, including, for example, systems based on a permanent magnet (PM) generator, an AC-AC converter, and a transformer. However, one drawback to these systems is their increased cost, due to the use of permanent magnets. More recently, still other approaches have been proposed in which AC power generated from a PM generator is converted to DC power for connection to a DC grid. These solutions likewise suffer from relatively high cost, due at least in part to the use of the PM generator. Accordingly, improved systems that have low cost and high performance are needed.